One of the major goals of the USPHS is the regional elimination of measles by the year 2000. Significant progress has been made in the past few years in the US due to an increase in immunization rates and implementation of a two dose schedule for measles containing vaccines. However, there are still some unanswered questions regarding optimal use of measles vaccines. For example, the preferred age for the second dose of measles vaccine was not known. In order to compare measles vaccination at 4-5 years of age with immunization at 11-12 years of age, 1800 children in Marshfied, Wisconsin received dose two of measles vaccine. Neutralizing antibody titers wer compared one and 6 months after immunization. All seronegative children were successfully immunized against measles. Children immunized at 4-5 years were more likely to achieve neutralizing antibody titers >900 (84%) than children immunized at 11-12 years of age. A follow-up study is curretly underway to see if these differences persist. These finding suggest that the primary effect of the measles second dose is to decrease the population of children with primary vaccine failure. There was little booster effect observed and no benefit derived to postponing immunization until adolescence. Two issues involved with the safety of measles vaccine were studied. Pestivirus contamination of measles vaccines had been reported in Japan. In order to see if other measles and viral vaccines contained pestivirus sequences we used PCR to screen for BVDV and HCV. One experimental vaccine was positive for BVDV sequences while all other viral vaccines tested were found to be negative. The use of measles containing vaccines may also be associated with post immunization immune thrombocytopenia. In order to determine the cause of post immunization thrombocytopenia, we have initiated a study to identify vaccine component(s) responsible for inducing anti-platelet antibodies after immunization with MMR.